[辨析]
because、as、since、for的用法：because表示原因的语气最强,常表示必然的因果关系,从句一般放在主句后面;另外,回答 why的问句只能用because. as表示一般的因果关系,语气比because弱,说明比较明显的原因,它引导的从句可以放在句首也可以放在句尾。since表示对方已经知道、无需加以说明的原因或事实。for是并列连词,语气较弱,用来补充说明理由或提供一种解释。如：He is not at school today because he is seriously ill./ As all of you have got here, now, lets go to the zoo./ I will ask Lin Tao to go with me since you are very busy./ We must be off now for the match starts at 7：00.
if、 whether的区别：表示是否时,if和whether同义,引导宾语从句,另外,whether还可以引导主语从句、表语从句 等名词性从句或者让步状语从句;而if还可以表示如果,引导条件状语从句, 。如：I dont know if/whether he will arrive on time./ I will ring you up if he arrives on time.
[注意]下列情况只能用whether不能用if：①引导主语从句,②引导表语从句,③引导从句作介词宾语,④引导不定式短语,⑤引导让步状语从句,⑥在动词discuss之后,⑦在wonder / not sure之后,⑧在if与whether含义易混时。如：Whether it is a fine day next Sunday is still a question. / Please ask him whether to go there with a raincoat or not. / Hainan is the place to be, whether its summer or winter./ Please let me know whether you need my help.